Cassettes for holding reels of magnetic recording tape are well-known, and videocassettes for home use are sold commercially in large volumes into a very price-competitive market. Videocassettes typically include a housing having a reel of magnetic recording tape called the supply reel and a take-up reel. One end of the tape on the supply reel is attached to the take-up reel. As shown in FIG. 1, the housing typically includes a lower half, called the base 10, and an upper half, called the cover (not shown). A door (not shown), which protects the exposed length of tape 16 running between the supply reel 18 and the take-up reel 20 when the cassette is not in use, is attached to the cover. These cassettes operate in a videocassette recorder (VCR) which is capable of both playing and recording.
The tape 16 follows a prescribed path from the supply reel 18, over various tape guides, out of the housing, across a planar area called the left bridge 22, across an open area 24 where the tape 16 is unsupported, to a planar area called the right bridge 26, and back into the housing to the take-up reel 20. The long side of the cassette on which the bridges 22, 26 are located is the front 28 of the cassette, and the long side opposite the front side of the cassette is the rear 30 of the cassette.
The end of the cassette nearest the supply reel 18 is the left end 32 of the cassette, and the end of the cassette nearest the take-up reel 20 is the right end 34. These conventions are consistent with the manner in which a videocassette is typically inserted into the VCR; the front 28 of the cassette enters the machine first, with the base 10 on the bottom and the rear 30 of the cassette trailing.
Several other parts are also included in the cassette which enable the entire device to be safely and conveniently placed in a compatible machine for recording or playing. These parts include reel brakes 36, 38, which pivot on pins 40, 42, which typically are molded as part of the base 10. The reel brakes 36, 38 keep the reels 18, 20 from unrolling the tape 16 when the cassette is out the VCR. Other safety devices signal the VCR that the end of the tape 16 has been reached, thereby preventing damage to the recorded material.
The base 10 and cover fit together to enclose tape reels 18, 20, along with the other internal parts, to exclude contaminants and protect the tape 16 from damage. Alignment of the cover with the base 10 is assured by screw bosses 44 in the base 10 which mate with corresponding bosses in the cover.
A significant part of the cassette cost arises from the size and complexity of the base 10 and cover. Videocassette housings typically are injection molded using a synthetic resin such as polystyrene or acrylonitrile-butadiene-styrene (ABS). Molds for the base 10 and cover are typically designed to pull apart vertically, with one half of the mold forming substantially all of the internal parts of the base 10 or cover, and the other half forming the outside surfaces of the part. Since videocassette housings have large wall areas, reducing wall thickness is important to minimizing injection molding costs. Molding the walls and the integral components contribute to part cost not only through the quantity of material used in the part, but also through the time required to cool the part before ejection from the mold. While material consumption is proportional to the first power of wall thickness, the cooling time required before a part can be ejected from a mold is more nearly proportional to the square of the wall thickness. Since cooling the part sufficiently to allow ejection without warping or other damage consumes about 80% of the total molding time, it is clear that reduction of wall thickness presents significant opportunities for increasing the productivity of the videocassette manufacturing process.
Another factor in determining wall thickness is resin flow during injection molding. During injection of the material into the mold, the various parts of the mold serve not only to determine the size and shape of the part being molded, but also act as passages through which the molten plastic must flow to reach other parts of the mold. If a wall section is too thin, the portion of the mold forming that wall section will be correspondingly thin, thereby restricting flow, and causing excessive reduction in injection pressure at points farther from the point of injection, or gate. As a result, the injection pressure may have to be raised, or the injection time may have to be lengthened, or both.
Increasing injection pressure can create undesirable side effects. U.S. Pat. No. 4,706,149 describes the ejection difficulties which can arise when injection pressures are high. Additionally, polymeric materials which are forced to flow through very thin channels at the high shear rates produced by high injection pressures often undergo excessive orientation of the polymer chains, which can weaken the part and directionally vary shrink rates, which can, in turn, lead to warping.
Since cassette bases and covers have sections having a variety of shapes and thicknesses, these various sections will require different times to cool during injection molding. The overall cooling time required for the part will be determined by the slowest cooling section, called the "critical section for cooling." Further, as the cooling time for some sections is shortened by thinning, it is possible that another section may become the critical section. Therefore, thinning of only the wall sections may not be sufficient to significantly shorten the overall cooling time for the part.
None of the known configurations for cassette bases and covers shorten the cooling time during injection molding for sections other than the wall sections.